Cryogenic condensation pump

ABSTRACT

The cryogenic condensation pump comprises a housing (3), accommodating a vessel (3) for a cryoagent, namely liquid nitrogen, a shell heat line and a chevron baffle which make up a radiation shield, and a pump-out element. The cryoagent vessel and pump-out element are provided with suspension pipes. Each of the suspension pipes is provided with assemblies, for joining it to the housing, and assemblies for joining it to the cryoagent vessel and the pump-out element. 
     The suspension pipe of the pump-out element is provided with a support bush, and in the cover of the cryoagent vessel an annular element is secured which makes contact with the support bush. The pump housing, radiation shield and pump-out element are made of metals having low specific weight.

FIELD OF THE INVENTION

The present invention relates to vacuum technology and more specificallyto the designs of cryogenic condensation vacuum pumps. The best use ofthe invention may be made in vacuum technology widely employed in theelectronic industry, radio engineering and other industries, as well asin those fields of research which need creating and maintaining, for along time, an ultrahigh-purity, completely oil-free ultrahigh vacuum inthe operating pressure range from 1×10⁻⁴ to 1×10⁻¹⁰ Pa.

DESCRIPTION OF THE PRIOR ART

At present, the improvement of cryogenic condensation pumps follows thepath of optimization of their designs with the purpose to reduce theirweight and metal consumption, simplify the processes of pump assemblyand disassembly, and improve the pump efficiency.

Known in the art is a cryogenic condensation pump containing a housing,accommodating a radiation shield comprised of a vessel for a cryoagent,a shell heat line and a chevron baffle that are properly interconnectedand a pump-out element in the form of a vessel. The vessel for thecryogent of the radiation shield and the pump-out element are providedwith suspension pipes serving to fill said vessels with cryoagents,respectively, liquid nitrogen and liquid helium, as well as to securethe vessels in the housing (M. P. Larin, "Pribory i TekhnikaEksperimenta", a journal of the Academy of Sciences of the USSR, Moscow,No. 2, 1982, pp. 130-133, cf. p. 132).

It is known that of primary importance in helium cryogenic condensationpumps is the problem of economically efficient consumption of liquidhelium for reasons of its scarcity and high price. However, in the pumpdescribed, the pump-out element made of copper and filled with liquidhelium experiences large heat inflows along a smooth-walled suspensionpipe. Therefore, in the pump described, relatively high vaporability ofthe liquid helium exists, with the result that this pump design is notsufficiently economical.

Also known in the art is a cryogenic condensation pump containing ahousing accommodating a radiation shield comprised of a vessel for acryogent, a shell heat line and a chevron baffle that are properlyinterconnected, and a pump-out element in the form of a vessel (SU, A,1017817). The pump-out element is in the form of a vessel and is locatedin the cavity formed by the bottom of the vessel for the cryoagent ofthe radiation shield, the surface of the shell heat line and the chevronbaffle. The vessel for the cryoagent of the radiation shield and thepump-out element are provided with suspension pipes serving to fill saidvessels with cryoagents, namely liquid nitrogen and liquid helium,respectively, as well as to secure these vessels in the housing. Toreduce heat inflows to the pump-out element filled with liquid heliumthrough a suspension pipe, the latter is made in the form of acorrugated metal tube with a helical corrugation profile. Suspensionpipes are joined by welding to the vessel for the cryoagent of theradiation shield or to the pump-out element. To join the suspensionpipes to the housing, their top ends are welded to the top ends of thehousing branch pipes into which the suspension pipes are inserted. Thepump housing and suspension pipes are made of stainless steel and theelements of the radiation shield, i.e., the cryoagent vessel, shell heatline and chevron baffle, as well as the pump-out element, of copper.

A disadvantage of this pump is that its assembly or disassembly presentsdifficulties, e.g., in repairs of the vessel for the cryoagent of theradiation shield or the pump-out element. In such cases, the top ends ofsuspension pipes and of housing branch pipes welded together must be cutand, upon completion of repair and subsequent reassembly of the pump,they must be rewelded so as to provide highquality welds of the pipeends. These operations are labour- and time-consuming and requirespecial conditions. Besides, pump elements of stainless steel and coppermean heavy weight and metal consumption, resulting in high stresses tothe pump element and their weld joints and thus causing the danger ofbreaking-off, especially in transit.

It should also be noted that the pump described is not sufficientlyeconomical on account of rather high consumption of cryoagents, i.e.,liquid helium and liquid nitrogen, because of their evaporation due tolarge heat inflows to the pump-out element and the vessel for thecryoagent of the radiation shield.

SUMMARY OF THE INVENTION

The principal object of the present invention is to provide a cryoageniccondensation pump comprising a housing accommodating a radiation shieldwith a cryoagent vessel and a pump-out element in the form of a vessel,provided with suspension pipes and in which the suspension pipes arejoined to the housing and the radiation shield vessel or to the pump-outelement. The housing, radiation shield and pump-out element are made ofsuch a material that assembly and disassembly of the pump become easierand simpler, the weight and the metal content of the pump are reducedand reliable vacuum-tight joints of pump elements are ensured.

With this principal object in view, there is proposed a cryogeniccondensation pump comprising a housing accommodating a radiation shieldcomprised of a vessel for a cryoagent, a shell heat line and a chevronbaffle that are properly interconnected, and a pump-out element in theform of a vessel, the vessel for a cryoagent of the radiation shield andthe pump-out element being provided with suspension pipes. In accordancewith the invention, every suspension pipe is provided at its oppositesides with assemblies for joining it to the housing and to the vesselfor a cryoagent of the radiation shield or to the pump-out element. Thesuspension pipe-to-housing joining assembly contain a branch pipelocated in a hole in the housing flange and hermetically secured withone of its ends on the suspension pipe, while at the other end of thebranch pipe a collar is provided with an annular projection facing thehousing flange, on which a matching annular projection is also provided,a sealing metal gasket being installed between said annular projections,while in the middle portion of said branch pipe an annular flange issecured, installed in which are stop bolts being in contact with thehousing flange. The assembly for joining a suspension pipe to the vesselfor a cryoagent of the radiation shield or to the pump-out element has abranch pipe with an end sealing collar hermetically joined to the neckof the vessel for a cryoagent of the radiation shield or the pump-outelement and two flanges joined together with their end surfaces facingeach other, one of which is hermetically attached at the end of asuspension pipe while the other is installed at the end of said branchpipe. The flanges are provided at their surfaces facing each other withannular projections. The end sealing collar of the branch pipe islocated between the annular projections of said flanges. The suspensionpipe of the pump-out element is provided with a support bush. In thecover of the vessel for a cryoagent of the radiation shield an annularelement is secured being in contact with the support bush. The pumphousing, radiation shield and pump-out element are made of metals havinglow specific weight.

The provision of each pipe with assemblies for joining it to the housingand to the vessel for a cryoagent of the radiation shield or to thepump-out element and executing these assemblies as described hereinabovefacilitates and simplifies assembly and disassembly of the pump whenrepairing the vessel for a cryoagent of the radiation shield or thepump-out element as may be required, e.g., in order to redepositaluminium film onto their surfaces. In disassembly of the pump, only thesuspension pipe-to-housing joining assemblies are disassembled,whereupon all the inside units of the pump are set free from thehousing. In assembling the units of the pump are mounted in a reverseorder.

Said joining assemblies manufactured as described above provide a meansto make the pump housing, radiation shield and pump-out element of ametal having low specific weight, e.g. the housing of titanium, theradiation shield and pump-out element of aluminium, which allowsreduction in the weight and metal content of the pump.

Such a design of joining assemblies as described above ensuresvacuum-tight connection of the pump housing made of titanium as well asof the radiation shield and the pump-out element made of aluminium withthe suspension pipes made of stainless steel. Due to the presence ofannular projections on the elements to be joined and of a sealing gasketbetween them made of a soft metal such as aluminium, tightening thejoining assemblies ensures vacuum-tight connection of the housing madeof titanium with a suspension pipe made of stainless steel and of thevessel for a cryoagent of the radiation shield or pump-out element madeof aluminium with a suspension pipe made of stainless steel.

It is expedient that the contact surfaces of the support bush andannular element be made conical. Such a construction of the contactsurfaces facilitates the process of dismantling the pump-out element indisassembly of the pump and provide for better thermal contact of themiddle portion of the suspension pipe with the vessel for a cryoagent ofthe radiation shield upon reassembly.

It is advantageous to furnish suspension pipe of the the pump-outelement, at the side adjacent to the pump-out element, with a shieldcoaxially installed and having thermal contact with the suspension pipenear the assembly for joining it to the pump-out element.

The presence of said shield reduces radiation heat inflow from thevessel of the radiation shield to the assembly for joining thesuspension pipe with the pump-out element held at liquid heliumtemperature and to the lower portion of the suspension pipe whosetemperature is close to liquid helium temperature. This reduces heatinflow by thermal conduction to the pump-out element itself from thesuspension pipe and the assembly for joining it to the pump-out element,thus reducing the vaporability of the liquid helium in the pump-outelement and hence enhances the economic efficiency of the pump.

It is advisable to provide the cryoagent vessel of the radiation shieldwith an additional shield installed between the housing and said vesselwith a clearance. The presence of an additional shield reduces radiationheat inflow from the pump housing to the cryoagent vessel, which, inturn, reduces the vaporability of the cryoagent, namely the liquidnitrogen, and thus enhances the economic efficiency of the pump.

It is advisable that a bush with coaxial blind holes at its oppositesides be installed at the bottom of the pump-out element and that thechevron baffle have a hole coaxial with the holes of the bush.

The presence of said bush with holes provides a possibility to securerigidly the pump-out element in the pump housing by means of rodsinserted into said holes and properly secured therein, which ensuresthat the pump elements and assemblies and their welded joints are intactin transit.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be further described in detail, using an example ofcarrying it into effect, with reference to the accompanying drawings,wherein:

FIG. 1 is a sectional view of the cryogenic condensation pump, accordingto the invention;

FIG. 2 is an enlarged sectional view of assembly A of FIG. 1;

FIG. 3 is an enlarged sectional view of assembly B of FIG. 1; and

FIG. 4 is an enlarged sectional view of assembly C of FIG. 1.

DETAILED DESCRIPTION OF THE INVENTION

The cryogenic condensation pump contains a housing 1 (FIG. 1) with acover 2 accommodating a vessel 3 for a cryoagent, namely liquidnitrogen, a shell heat line 4 and a chevron baffle 5 which make up aradiation shield 3, 4, 5, and a pump-out element 6. The cryoagent vessel3 has a cover 7 and a bottom 8. The pump-out element is made in the formof a vessel filled with a cryoagent, liquid helium, and is located inthe cavity formed by the bottom 8 of the cryoagent vessel 3, the shellheat line 4 and the chevron baffle 5.

The cryoagent vessel 3 of the radiation shield 3, 4, 5 and the pump-outelement 6 are provided with appropriate suspension pipes 9 and 10. Thesuspension pipe 9 has at its opposite ends assemblies 11a and 12a forjoining it to the housing 1 and to the cryoagent vessel 3, respectively.The suspension pipe 10 also has at its opposite ends assemblies 11b and12b for joining it to the housing 1 and to the pump-out element 6,respectively.

The assembly 11a for joining the suspension pipe 9 to the housing 1contains a branch pipe 13 (FIG. 2) located in a hole 14 of a flange 15of the housing 1. The flange 15 is located inside a branch pipe 16 ofthe cover 2 of the housing 1 and is rigidly secured to the branch pipe16 and thus to the housing 1.

One end 17 of the branch pipe 13 is hermetically fixed on the suspensionpipe 9 while its other end has a collar 18 with an annular projection 19facing the flange 15 of the housing 1, on which a matching annularprojection 20 is provided. A sealing gasket 21 made of a soft metal,aluminium, is installed between the annular projections 19 and 20. Inthe middle portion of the branch pipe 13 an annular flange 22 isinstalled by thread connection. This flange is provided with threadedholes into which stop bolts 23 are inserted.

The assembly 11b for connecting the suspension pipe 10 to the housing 1is made similarly to the assembly 11a for connecting the suspension pipe9 to the housing 1 with the only difference that the flange 15 issecured directly to the cover 2 of the housing 1 and not shownseparately to avoid complication of the drawings.

In the assemblies 11a and 11b for joining the suspension pipes 9 and 10,respectively, to the pump housing 1, tightening the stop bolts 23ensures vacuum-tight connection of the housing 1 made of stainless steeldue to the annular projections 19 and 20 on the surfaces facing eachother, respectively, of the collar 18 and the flange 15 of the housing 1and the sealing gasket 21 made of a soft metal, aluminium, arrangedtherebetween.

The assembly 12a for joining the suspension pipe 9 to the cryoagentvessel 3 of the radiation shield 3, 4, 5 contains a branch pipe 24 (FIG.3) with an end sealing collar 25 hermetically joined to the neck 26 ofthe cryoagent vessel 3. The assembly 12a also has two flanges 27 and 28facing each other by their end surfaces 29 and 30, respectively, andjoined together by tie bolts 31. The flange 27 is hermetically fixed atthe end of the suspension pipe 9, while the flange 28 is installed atthe end of the branch pipe 24. The flanges 27 and 28 are provided, attheir end surfaces 29 and 30, respectively, with annular projections 32and 33, respectively, while an end sealing collar 25 is located betweenthem.

The assembly 12a for joining the suspension pipe 9 to the cryoagentvessel 3 is provided with a shield 34 having a collar 35. The shield 34reduces radiation heat inflow from the pump housing 1 to the assembly12a.

The assembly 12b for joining the suspension pipe 10 to the pump-outelement 6 is made similarly to the assembly 12a for connecting thesuspension pipe 9 with the cryoagent vessel 3 and is shown in FIG. 4where the respective elements are designated by the same referencenumerals as in FIG. 3.

In the assemblies 12a and 12b for joining the suspension pipes 9 and 10to the cryoagent vessel 3 and to the pump-out element 6, respectively,tightening the tie bolts 31 ensures vacuum-tight connection of thecryoagent vessel 3 or the pump-out element 6 which are made of aluminiumto the suspension pipes 9, 10 made of stainless steel, due to thepresence of projections 32 and 33 on the end surfaces 29 and 30 facingeach other or the flanges 27 and 28 and the sealing collar 25 of a softmetal, aluminium, arranged therebetween.

The suspension pipe 10 of the pump-out element 6 passes through thecryoagent vessel 3 of the radiation shield 3, 4, 5 inside a cylinder 36installed along the axis of the cryoagent vessel 3 and fixed in thecover 7 and bottom 8 of said vessel. The suspension pipe 10 is providedwith a support bush 37 which is screwed onto the helical corrugations ofthe suspension pipe 10 while an annular element 38 which is in contactwith the support bush 37 is fixed in the cover 7 of the cryoagent vessel3. The contact surfaces of the support bush 37 and annular element 38are made conical. Such a construction of the contact surfaces of thesupport bush 37 and annular element 38 facilitates the process ofmounting and dismantling the pump-out element 6 in assembly anddisassembly of the pump. Besides, this ensures better thermal contact ofthe suspension pipe 10 with the cryoagent vessel 3, thus reducing heatinflows by thermal conduction along the upper portion of the suspensionpipe 10 from the pump housing 1.

The cryoagent vessel 3 is provided with an additional shield 39installed between the housing 1 and the cryoagent vessel 3 with aclearance 40. The shield 39 is hung onto the collar 35 of the shield 34(FIG. 3). The presence of this shield reduces radiation heat inflowsfrom the pump housing 1 to the cryoagent vessel 3, which reduces thevaporability of the cryoagent, liquid nitrogen, and thus enhances theeconomic efficiency of the pump.

The suspension pipe 10 of the pump-out element 6 is provided, at itsside adjacent to the pump-out element, with a coaxially installed shield(FIG. 4) consisting of two portions, the upper one 41 embracing thesurface of the suspension pipe 10 at half its length and the lower one42 protecting the assembly 12b for joining the suspension pipe 10 to thepipe-out element 6. The upper portion 41 and the lower portion 42 of theshield are joined together by thread 43. The shield has a thermalcontact with the suspension pipe 10 in the vicinity of the joint to thepump-out element 6. Contact is made by means of a collar 44 provided onthe lower portion 42 of the shield and engaging the helical corrugationsof the suspension pipe 10.

The end of the suspension pipe 10 is directly joined by welding to theflange 27 of the assembly 12b for joining it to the pump-out element 6,and hence has a temperature close to the liquid helium temperature inthe pump-out element 6 (4.2K). Due to the thermal contact of thesuspension pipe 10 with the lower portion 42 of the shield, thetemperature is kept at a value close to the liquid helium temperaturealong the shield. The temperature of the suspension pipe 10 varies fromits end adjacent to the pump-out element 6 to the midlength between 4.5Kand 50K.

The presence of the shield reduces radiation heat inflow from the vessel3 of the radiation shield to the assembly 12b for joining the suspensionpipe 10 to the pump-out element 6 and to the lower portion of thesuspension pipe 10. This reduces heat inflow by thermal conduction tothe pump-out element 6 from the suspension pipe 10 and the assembly 12bfor joining it to the pump-out element 6, thus reducing the vaporabilityof the liquid helium and enhancing the economoc efficiency of the pump.

The pump is provided with flanges 45, 46, 47 for connection to a boosterhigh-vacuum pump, e.g., of the cold-cathode ion type, to a force-pump,e.g., of the sorption type, and the suction chamber (not shown in thefigures to avoid complicating the drawings), and a blank flange 48 witha bottom 49 serving to carry the pump and test it alone, connected toitself.

In the bottom 50 of the pump-out element 6, along its axis, a bush 51 isfixed having, at its opposite ends, blind holes 52 and 53. In thechevron shield 5 there is provided a hole 54 coaxial with holes 52 and53. In transit, inserted into the hole 52 is a rod which is passedthrough the suspension pipe 10 and is fixed to the branch pipe 13. Intothe hole 53 is inserted a rod which is passed through the hole 54 in thechevron baffle 5 and is secured in the bottom 49 of the blank flange 48(the rods are not shown in the figures to avoid complicating thedrawings).

The bush 51 with holes 52, 53 provides a means to secure rigidly thepump-out element 6 using rods, which ensures that the elements andassemblies of the pump are intact in transit.

The pump housing 1 is made of titanium while the radiation shieldcomprised of the cryoagent vessel 3, the shell heat line 4 and thechevron baffle 5, and the pump-out element 6 are made of aluminium. Thisallows reduction in the weight and the metal content of the pump.

The cryogenic condensation pump operates as follows.

At the arrival of the pump, the blank flange 48 is removed, the lowerrod is pulled out and a copper or aluminium plug is screwed into thehole 54. The pump is installed with the flange 47 on the matching flangeof the suction chamber, the flanges being connected vacuum-tightly. Theupper rod is pulled out. Then the plugs are removed from the flanges 45and 46, and a cold-cathode ion pump is connected to the flange 45, whilea valve with a metal seal is connected to the flange 46. Through thisvalve, a fore-pump system consisting of a mechanical fore-pump and asorption pump is connected to the pump. The space to be evacuated in theproposed pumped and the suction chamber are first pump down to apressure of 100 to 40 Pa by the mechanical fore-pump, and then down to apressure of 1×10⁻² to 1×10⁻⁴ Pa by the sorption pump.

Thereupon, the cryoagent vessel 3 is filled with liquid nitrogen throughone of the suspension pipes 9. Once the pressure in the pump has beenreduced by an order of magnitude, the cold-cathode ion pump is switchedon and the pressure in the pump is reduced by another order of magnitudeor two.

Now, it is expedient to preheat the suction chamber to 200° to 250° C.,taking care that its pressure does not rise above 1×10⁻⁴ Pa. As a resultof such a four-to-eight hours heating of the suction chamber itspressure, upon cooling, usually falls down to 1×10⁻⁶ to 1×10⁻⁷ Pa.

Next, to save liquid helium, the pump-out element 6 is cooled down usinga small quantity of liquid nitrogen from 1 to 2 l, to a temperature of80 to 100K, by filling it through the suspension pipe 10. In so doing,the temperature may be monitored by a thermo-couple immersed along thesuspension pipe 10 down to the bottom of the pump-out element 6.

Upon completing this operation, the cavity of the pump-out element 6should be evacuated, through the suspension pipe 10, by the mechanicalfore-pump down to a pressure of 100 to 40 Pa, and the pump-out element 6should be filled with gaseous helium and then with liquid helium. As aresult, the pressure in the suction chamber usually falls down to 1×10⁻⁷to 1×10⁻⁹ Pa or even below. Next, the branch pipe 13 is connected to thegaseous helium collect system. The pump thus prepared is used toevacuate the suction chamber down to a required pressure.

Industrial Applicability

The present invention may be used to best advantage in vacuum technologywidely employed in the electronic industry, radio engineering and otherindustries, as well as in those fields of research which need creatingand maintaining, for a long time, an ultrahigh-purity, completelyoil-free ultra high vacuum in the operating pressure range from 1×10⁻⁴to 1×10⁻¹⁰ Pa.

What is claimed:
 1. A cryogenic condensation pump comprising a housingmade of a metal having low specific weight; a radiation shieldaccommodated in said housing, made of a metal having low specific weightand comprised of a cryoagent vessel, a shell heat line and a chevronbaffle properly interconnected; a pump-out element in the form of avessel made of a metal having low specific weight and located in acavity defined by a bottom of said cryoagent vessel, said shell heatline and said chevron baffle; first and second suspension pipesconnected, respectively, to the cryoagent vessel and the pump-outelement;assemblies for joining said first and second suspension pipes tosaid housing, each of said assemblies comprising a flange on saidhousing with an annular projection; a branch pipe located in a hole insaid flange in said housing and having a first end hermetically securedon said suspension pipe; a collar provided on a second end of saidbranch pipe and having an annular projection facing the flange on saidhousing and said annular projection matching said annular projection onsaid flange of said housing; a sealing metal gasket installed betweensaid annular projections of said collar and said flange of said housing;an annular flange secured in a middle portion of said branch pipe; andstop bolts installed in said annular flange in said middle portion ofsaid branch pipe and said stop bolts being in contact with said flangeof said housing; assemblies for joining said first and second suspensionpipes to the cryoagent vessel or to the pump-out element, each assemblycomprising a branch pipe having an end sealing collar hermeticallyjoined to a neck of the cryoagent vessel or the pump-out element; firstand second flanges joined together and having end faces facing eachother, said first flange being hermetically attached at a second end ofthe suspension pipe and the second flange being installed at a end ofsaid branch pipe, the surfaces of said flanges facing each other beingprovided with annular projections and said end sealing collar beinglocated between the annular projections of said flanges; a support bushprovided on the surface of said second suspension pipe; and an annularelement in contact with said support bush and being fixed in a cover ofsaid cryoagent vessel.
 2. A pump as claimed in claim 1 wherein contactsurfaces of the support bush and the annular element are made conical.3. A pump as claimed in claims 1 or 2 wherein the second suspension pipeof the pump-out element is provided, at the side adjacent to thepump-out element, with a coaxially installed shield having thermalcontact with the second suspension pipe in the vicinity of the assemblyfor joining it to the pump-out element.
 4. A pump as claimed in claim 1wherein the cryoagent vessel of the radiation shield is provided with anadditional shield installed between the housing and said cryoagentvessel with a clearance between them.
 5. A pump as claimed in claim 1further comprising, in a bottom of the pump-out element, along its axis,a bush with coaxial blind holes at its opposite ends, and wherein in thechevron baffle a hole is made coaxial with the holes in the bush in thebottom of said pump-out element.